An experimental formulation of Bacillus thuringiensis var. israelensis was used in the laboratory to assess the influence of water temperature and pH on the relationship between concentration, duration of exposure, and mortality of the northern black fly species Simulium decorum and Prosimulium mixtum/fuscum group. Mortality increases in both species with increases in duration of exposure, concentration, temperature and pH. Onset of death is shortened by increase in concentration and temperature. As temperature rises, the concentration of B.t.i. required to induce mortality decreases; the sharpest decline occurring between 12 and 18 degrees C for S. decorum, and between 4 and 8 degrees C for P. mixtum/fuscum larvae. Lower pH induces a loss of efficacy of the B.t.i. formulation on S. decorum larvae at 4 and 12 degrees C. Dialysis of the B.t.i. formulation at pH 11 for 2 h increases its potency against S. decorum larvae, suggesting an effect of an extralarval alkaline hydrolysis on the B.t.i. efficacy. An alkaline prehydrolysis of the paracrystalline bodies could therefore be used in cold and acidic environments to compensate for loss of efficacy.